1. Field of the Invention
The invention relates generally to an apparatus for supporting printed circuit boards during manufacture and, more particularly, to a thin support for supporting boards as they are transferred through inline pass through ovens.
2. Description of Related Art
Inline pass through ovens are often used to manufacture printed circuit boards. Such ovens have been used, for example, to solder surface mount devices to a board or to cure adhesives securing components to a board. Typically, to solder surface mount devices, solder paste is screen printed onto the circuit board in desired locations. The surface mount components are then appropriately placed in the solder paste. A part of the oven called a transfer system, typically supports the "parallel edges" of the board while it conveys the board through the oven. The inline pass through oven heats the board and solder, soldering the components to the board. "Parallel edges" refers to the edges of a printed circuit board that are basically parallel to direction that the transfer system moves the circuit board. Inline pass through ovens that solder components are often called reflow ovens.
Some conventional transfer systems support the "parallel edges" of a printed circuit board with what we will call "edge supports. " An edge support may comprise support rails, chains and pins extending from the chains. Support rails such as these have been used in ovens and transfer systems manufactured by a company called Research Incorporated, located in Minneapolis, Minn., for example.
Edge supports are typically constructed by fixing support rails to a frame of the pass through oven. Chains typically rotate through these support rails, and pins typically extend from these chains such that a printed circuit board can rest on the pins. The pins typically extend 0.1-0.2 inch beneath the circuit board's parallel edges. As the chains rotate through the support rails, they move the pins such that the chains and pins carry circuit boards through the oven. We will refer to the process of conveying a board along a manufacturing line (e.g. through an oven) as "transferring" the board.
Reflow ovens typically use a specific thermal profile to solder components. For example, a reflow oven may have 4 or more temperature zones through which the board travels. The first 4 or 5 zones, for example, typically bring a board up to a desired temperature in a controlled manner. The final zone, called a spike zone, rapidly brings the board up to reflow temperatures, typically in the range of 183.degree.-210.degree. Celsius, soldering the components to the board. Inline pass through ovens used for purposes other than solder reflow often operate in a similar manner, but may use different thermal profiles.
The transfer systems used with some conventional inline pass through ovens support only the parallel edges of the printed circuit boards. Depending upon the weight of the components on a printed circuit board and/or the thickness of the board, however, the elevated thermal profile used with reflow or other types of inline pass through ovens may cause the circuit board to sag. Sagging can occur because boards, sometimes made of epoxy-glass laminates, may go through a glass transition phase and become weak at certain temperatures. For example, a widely available epoxy-glass laminate that has been used in printed circuit boards, FR4, typically has a transition phase around 150.degree. C. Generally, the higher the temperature above this transition temperature, the weaker a board becomes. Sagging can result in warped boards that may not fit into their final assembly. It can result in open solder joints because the solder cannot bridge the gap between the component and the warped board. Extreme warping can result in boards falling off the transfer system. Accordingly, parallel edge support, alone, may be adequate for some rigid printed circuit boards, but additional support may be required to control sagging of thin printed circuit boards or printed circuit boards with heavy components.
Conventional approaches to preventing sagging have added a dynamic or moving support between the edge supports. One conventional approach has used a steel cable to provide this additional support. This approach attaches the cable to a drive system. The cable's drive system causes the cable to move with the chains and pins of the edge supports as they carry the board through the oven.
Steel cable, however, has an inherent tendency to sag from its own weight. To insure a straight support surface, this approach must apply a high tensile force to the cable. Additionally, because of space constraints, the drive system may force the cable through tight radii. Tight radii in a dynamic system combined with the high tensile force can accelerate cable fatigue and failure. Accordingly, conventional dynamic steel cable systems have failed to provide an optimum solution to the sagging problem.
Another conventional approach to preventing sagging adds a support rail and roller chain between the edge supports. Further, the thermal characteristics of the rail and chain can affect the heating of the board. In fact, conventional systems have employed methods for heating support rails to prevent heating problems caused by the thermal characteristics of such rails. Such heating methods potentially add even further cost and complexity to the system. An additional rail and roller chain between the edge supports can also limit printed circuit board layouts by requiring a wide contact area (an area where no components may exist) on the bottom of the circuit board where the roller chain supports the board. Components in this area might be dislodged or damaged by the rail or roller chain. Accordingly, dynamic rail and roller chain systems have also failed to provide an optimum solution to the sagging problem.
There has been a need for a support system that does not have the complexity of conventional dynamic systems. There has been a need for a support system less sensitive to the fatigue or durability problems of conventional dynamic systems. There has been a need for a system that will have minimal impact on printed circuit board layouts and the thermal characteristics of the heating process. There has been a need for a system that prevents sagging using a sag support that does not have to move during transfer. The present invention meets these needs.